I'll start working on a new mode for the calculator now, a ratio conversion. A mode that keeps the "distance" your crosshair moves constant. The crosshair never really moves, but you get the idea
Four years back when I started this, converting 360 degree turn seemed to be the most universal standard for sensitivity conversion. Later the desktop-to-game conversion came, which in a sense keeps the ratio constant. There are however two problems with this conversion:
The distance covered by the mouse pointer in Windows (let's call it X) can never be perfectly matched to moving the crosshair to an object X pixels away all over the screen. This is because the view in an FPS or 3rd person game is spherical, while the view in Windows is obviously two dimensional. The best you can do is match it for one fixed parameter, and I chose moving the cursor from the center of the screen to the edge to match turning to an object at the edge of the screen in-game. This makes short crosshair movement around the center a little bit off, while larger movements towards the edge of the screen match good. The second problem is that you can't convert from one game to another with this mode, you need to base them all off of the Windows / Desktop input.
Now with a ratio conversion (ratio being the correlation between how far you physically have to move the mouse to aim at an object in-game, regardless of FOV), the second problem will be solved. But the first problem is still there to a certain degree, as you have to match the ratio at a fixed point. And I plan to add this as an option if possible, so you choose where you want the ratio to match. For instance a percentage between the crosshair and the edge of the screen.
No promises of what will come yet as I haven't started coding, but something like this is the plan.